Energy Released by Fuels
- We can experimentally determine the amount of energy released by a fuel
- We do this using calorimetry
- The apparatus for this experiment is shown below
Diagram to show the set up of calorimetry equipment
A lid is used to prevent heat loss
- The steps are:
- Measure a fixed volume of water into a copper can
- Weigh the spirit burner containing a fuel using a balance
- Measure the initial temperature of the water
- Burn the fuel and stir the water
- Wait until the temperature has risen by approximately 20 oC and extinguish the flame
- Record the final temperature of the water and re-weigh the spirit burner
- To calculate the energy released by the fuel we can use the data obtained from the experiment above and the specific heat capacity of water
- The specific heat capacity, c, is the energy needed to raise the temperature of 1 g of a substance by 1 °C
- The specific heat capacity of water is 4.18 J/g/°C
- The heat energy change is calculated using:
Q = m x c x ΔT
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- Q = the heat energy change, J
- m = the mass of the substance being heated, g
- c = the specific heat capacity, J/g/°C
- ΔT = the temperature change, °C
Worked example
1.023 g of the fuel propanol was burned in a spirit burner and used to heat 200 g of water in a copper calorimeter.
The temperature of the water rose by 30 oC.
Calculate the heat energy change, Q, for the combustion of propanol using this data.
Answer:
- Q = m x c x ΔT
- Q = 200 g x 4.18 J/g/°C x 30 °C = – 25 080 J
- The negative energy change indicates the reaction is exothermic
Examiner Tip
You do not need to be able to recall the equation, Q = mcΔT as you will be provided with it in the exam.
You must be able to use it to calculate the energy released by a fuel using the data provided.
